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Abstract:

A field device for process instrumentation comprising a measurement
transducer having an analog output to which a two-wire line can be
connected for transmitting an analog output signal, wherein a unit
galvanically separates the processor from an analog output circuit. A
read-back channel for a pulse-width modulated digital signal, which is
tapped behind an element for galvanic separation, is provided for
monitoring generation of the analog output signal, and a unit is provided
for monitoring the analog output signal to maintain a specifiable maximum
deviation from a reference signal, which is generated in the same manner
as the analog output signal by low-pass filtering from the pulse-width
modulated digital signal. In the case of transient signals, the
monitoring unit advantageously detects error states quickly and
appropriate measures for reacting to the error states can therefore be
implemented in less time.

Claims:

1.-3. (canceled)

4. A field device for process instrumentation, comprising: an analog
output, to which a two-wire line for transmitting an analog output signal
is connectable; a computation unit; a generation device configured to
generate a first pulse-width-modulated digital signal corresponding to
the analog output signal; a DC-isolation device configured to receive the
first pulse-width-modulated digital signal and output a second
pulse-width-modulated digital signal corresponding to the first digital
signal; a first low-pass filter configured to generate the analog output
signal from the second pulse-width-modulated digital signal; a first
monitoring device configured to monitor the generation of the analog
output signal, the first monitoring device comprising: a read-back
channel configured to return the second pulse-width-modulated digital
signal, through the DC-isolation device, to the computation unit as a
read-back signal in order to be checked; a second low-pass filter
configured to generate a comparison signal for the analog output signal
from the second pulse-width-modulated digital signal; and a second
monitoring device configured to monitor the analog output signal for
compliance with a predefinable maximum deviation from the comparison
signal and to generate an indication signal if the a predefinable maximum
deviation is exceeded.

5. The field device as claimed in claim 4, wherein the second monitoring
device comprises: a comparator configured to generate the indication
signal, wherein the indication signal is passed to the computation unit
for further handling of a detected error state.

6. The field device as claimed in claim 4, further comprising: a former
device configured to form an output signal by superimposing a
frequency-modulated signal in accordance with a HART protocol on the
analog output signal; and wherein the first monitoring device further
comprises a subtractor configured to remove the superimposed
frequency-modulated signal from the analog output signal.

7. The field device as claimed in claim 5, further comprising: a former
device configured to form an output signal by superimposing a
frequency-modulated signal in accordance with a HART protocol on the
analog output signal; and wherein the first monitoring device further
comprises a subtractor configured to remove the superimposed
frequency-modulated signal from the analog output signal.

8. The field device as claimed in claim 4, wherein the field device
comprises a measurement transducer.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a U.S. national stage of application No. PCT/EP2010/066108
filed 26 Oct. 2010. Priority is claimed on German Application No. 10 2009
050 645.4 filed 26 Oct. 2009, the content of which is incorporated herein
by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a field device for process instrumentation
and, more particularly, to a measurement transducer having an analog
output to which a two-wire line for transmitting an analog output signal
can be connected.

[0004] 2. Description of the Related Art

[0005] Field devices for process instrumentation are often used in
automation technology, where the field devices are used, for example, to
record and/or influence process variables and are connected to one
another by an automation network for interchanging data. Field devices
that record a physical or chemical variable as a process variable are
often referred to as measurement transducers because they convert the
respective variable into a measured value and output the latter to a
superordinate control station, for example, or, in the form of an actual
value, to a controller for further processing. Examples of such
measurement transducers are measurement transducers for a filling level,
mass flow, pressure, temperature, pH or conductivity.

[0006] EP 1 192 614 B1 discloses a measurement transducer that converts a
physical or chemical measurement variable into an analog output signal
that corresponds to the measured value and which can be transmitted on a
two-wire line. For this purpose, a suitable sensor for the physical or
chemical measurement variable, a downstream analog/digital converter, a
computation unit subordinate to the latter and an output circuit that is
controlled by the computation unit and can be connected to the two-wire
line are provided. The sensor converts the measurement variable into a
sensor signal which is digitized in the analog/digital converter and is
preprocessed in the computation unit to form a desired value which is
used to set the analog output signal on the two-wire line by a control
device in the output circuit. The digital desired value, in the form of a
predefined current value, is first of all converted into an analog
desired current value in the output circuit using a digital/analog
converter and is converted into an analog loop current by a downstream
control circuit. The actual value of the loop current is recorded using a
measuring element for current/voltage conversion. Low-impedance current
measuring resistors, which are also referred to as shunts, can be used as
simple measuring elements for this purpose. The voltage signal which is
generated thereby and is proportional to the actual value of the loop
current is supplied to a comparison device for the purpose of forming a
control deviation from the analog desired current value and the actual
value of the loop current.

[0007] A controller comprising an analog controller having an integrated
operational amplifier uses the control deviation to determine, for the
purpose of correcting the control deviation, an actuating signal for an
actuator that is formed by a transistor circuit for setting the loop
current. At the same time, the measured analog actual value of the loop
current is digitized with the aid of an analog/digital converter and is
supplied to the computation unit in the form of a digital actual value.
The computation unit is thus able to determine deviations between the
digital actual value and the digital desired value and to respond thereto
by appropriate tracking of the desired value. If impermissibly large
deviations occur, the user of the field device is informed of this. This
may be effected via a data interface and/or a display apparatus for
transmitting and/or displaying an item of information relating to the
deviation which has been determined and thus relating to the measurement
error. The data interface is part of the output circuit and enables data
communication via the two-wire line according to the Highway Addressable
Remote Transducer (HART) protocol.

[0008] Data communication according to the HART protocol is performed
using continuous-phase frequency modulation with a data transmission rate
of 1200 bits/sec. The frequency-modulated current signal, the amplitude
of which is supposed to be less than 600 μA, is superimposed on the
analog output signal from a 4 to 20 mA interface in a known manner.

[0009] DE 10 2007 059 847 A1 discloses a field device having an analog
output, in which it is possible to detect a malfunction of the controller
or a malfunction of the actuator for the loop current. For this purpose,
the actuating signal is monitored for compliance with a predefinable
range of values with the aid of a comparator. If a deviation from the
range of values occurs, an indication signal signals the detected error
state and suitable error-handling measures may be initiated. This may be,
for example, the output of an error message or the notification of a
superordinate control station of a need for maintenance via the two-wire
line and/or the assumption of a safety state by the field device. A
safety state is indicated in the analog output signal by a current level
that is greater than 22.6 mA or less than 3.5 mA.

[0010] One problem when monitoring an analog output circuit for correct
operation is represented by the low-pass filter that usually has to be
used to smooth the analog output signal. In particular, if a favorable
pulse width modulator with a downstream low-pass filter is used for
digital/analog conversion, the low-pass filter, which is intended to
limit the analog output signal to a bandwidth of less than 25 Hz,
constitutes a considerable part of the analog output circuit. Removing
the frequency-modulated signal that has been superimposed for data
transmission is also problematic when monitoring the analog output
current for a correctly set value if a low-pass filter is used for this
purpose. Low-pass filters always give rise to a signal delay, with the
result that a current value measured at the output for monitoring
purposes has a considerable delay with respect to a digital value which
is applied to a digital/analog converter of the analog output circuit in
order to generate the analog output signal. In addition, low-pass filters
usually do not have a constant group delay time, with the result that
transient signals in the transition area are output in a corrupted
manner. The signal delay in the analog output circuit therefore cannot be
easily compensated for by a corresponding delay of the digital value,
which is passed to the digital/analog converter, in a monitoring device.
One possibility for circumventing the described problems is to exclude
time windows with transient signal profiles from the monitoring process.
However, a monitoring process would then presuppose a desired value of
the output current which is constant over a minimum period of time.
However, since it cannot be ensured that this condition is met in a
predefined period of time, a maximum period of time in which an error
state of the analog output circuit is detected therefore could not be
guaranteed.

SUMMARY OF THE INVENTION

[0011] It is therefore an object of the invention to provide a field
device for process instrumentation having an analog output to which a
two-wire line for transmitting an analog output signal can be connected,
where it is possible to monitor the circuit for generating the analog
output signal with improved accuracy and/or with lower demands imposed on
the signal profile.

[0012] This and other objects and advantages are achieved in accordance
with the invention by providing a field device in which the process of
monitoring the analog output circuit can be constantly activated, and
time windows in which the monitoring process is interrupted are virtually
not required. Malfunctions of the analog output circuit can thus be
detected without a significant delay. As a result, a field device can be
changed to a safe state in a comparatively quick manner in the event of a
malfunction.

[0013] The monitoring process can advantageously be used with any type of
signal profile, for example, with transitions in the form of steps,
staircases or ramps, as well as with a constant profile of the analog
output signal.

[0014] Since only digital signals can be passed through a DC-isolation
device, the outlay for analog DC-isolation in the analog output circuit
is advantageously dispensed with. As for the outlay for producing the
analog output circuit, it is also advantageous that it is possible to
dispense with an expensive digital/analog converter since the conversion
is performed with the aid of pulse width modulation and subsequent
low-pass filtering. Since the filter is implemented in the analog part of
the analog output circuit, there is also no need for any complex software
which would take up a large part of the computation power available in
the computation unit.

[0015] In the field device in accordance with the invention, it is
possible to completely monitor the circuit for generating the analog
output signal in a particularly advantageous manner. This is because
errors in the DC-isolation device, which would not be detected solely by
monitoring the analog output signal for compliance with a predefinable
maximum deviation from the comparison signal, are also detected with the
aid of the read-back channel for the pulse-width-modulated digital
signal.

[0016] In one particularly advantageous embodiment, a comparator is
provided for the purpose of generating the indication signal, and the
indication signal is passed to the computation unit for further handling
of a detected error state. This has the advantage that, if an error state
is detected, the computation unit can immediately initiate suitable
error-handling measures. These may be the same measures which are also
taken when the read-back channel of the pulse-width-modulated digital
signal detects an error. Here, it is advantageously possible to dispense
with an additional input of the computation unit for the indication
signal and an additional channel in the DC-isolation device if, when an
indication signal is generated, the indication signal interrupts the
read-back channel.

[0017] In another advantageous embodiment, in the case of a field device
in which a frequency-modulated signal in accordance with the HART
protocol is superimposed on the analog output signal, the device for
monitoring the generation of the analog output signal may be supplemented
with a subtractor that removes the superimposed frequency-modulated
signal from the output signal and is connected upstream of the device for
monitoring the analog output signal for compliance with the predefinable
maximum deviation in the path of the analog output signal. As a result,
the monitoring device can be equally used in 4 to 20 mA interfaces with
or without HART communication and the maximum deviation can be predefined
such that the maximum deviation is less than the amplitude of the
superimposed frequency-modulated signal.

[0018] Other objects and features of the present invention will become
apparent from the following detailed description considered in
conjunction with the accompanying drawings. It is to be understood,
however, that the drawings are designed solely for purposes of
illustration and not as a definition of the limits of the invention, for
which reference should be made to the appended claims. It should be
further understood that the drawings are not necessarily drawn to scale
and that, unless otherwise indicated, they are merely intended to
conceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention as well as refinements and advantages are explained
in more detail below using the drawing which illustrates an exemplary
embodiment of the invention, in which:

[0020] The FIGURE is a schematic block diagram of the field device in
accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The figure shows the fundamental structure of a measurement
transducer 1 which is suitable for use in an automation system for
recording a chemical or physical variable 2. The variable 2 is passed to
a sensor 3 which converts the variables into an electrical signal. The
analog measurement signal generated in this manner is digitized using an
analog/digital converter 4 which is located at an analog input of a
computation unit 5 in which the linearity and temperature behavior of the
measurement signal are corrected. A digital predefined value of an analog
output signal 7, which value corresponds to the measured value and is
calculated by the computation unit 5, is converted into the analog output
signal 7 with the aid of a digital/analog converter. The digital/analog
converter is formed by a pulse width modulator 8 and a downstream
low-pass filter 9. The pulse width modulator 8 may comprise a component
that is separate from the computation unit 5 or may be integrated in the
computation unit 5 if the computation unit 5 itself is able to generate
and output a pulse-width-modulated digital signal corresponding to the
digital predefined value. The low-pass filter 9 limits the bandwidth of
the analog output signal 7 to a frequency range below 25 Hz.

[0022] The circuit shown for generating a loop current I in a two-wire
line 10, 11 is provided with a device for DC-isolating an analog output
30 from circuit parts 31 with the sensor 3, which device consists of four
elements 12a, 12b, 12c and 12d. Each of these elements is used for the
DC-isolated transmission of a digital signal. The element 12b is
connected between the pulse width modulator 8 and the low-pass filter 9.
In order to monitor the pulse width modulator 8 and the element 12b,
provision is made of a read-back channel which is used to pass a second
pulse-width-modulated digital signal 13, which is tapped off downstream
of the element 12b, to the computation unit 5 via the element 12c of the
DC-isolation device as a read-back signal 14. The read-back signal 14 is
a digital value which is obtained with the aid of a demodulator 15 in
accordance with the second pulse-width-modulated digital signal 13.

[0023] A second low-pass filter 16, which preferably has an identical
design to the low-pass filter 9, is used to monitor the correct operation
of the low-pass filter 9. The second low-pass filter 16 provides a
comparison signal 17 which is compared with a measured analog output
signal 7' corresponding to the analog output signal 7 with the aid of a
subtraction element 1B and a comparator 19. If the deviation of the
measured analog output signal 7' from the comparison signal 17 exceeds a
predefinable maximum value, an indication signal 20 which indicates an
erroneous state is generated. So that there is no need for an additional
channel in the DC-isolation device for the purpose of supplying the
indication signal 20 to the computation unit 5, the indication signal 20
is passed to the element 12c in a manner linked to the second
pulse-width-modulated digital signal 13. If there is a level of the
indication signal 20 to indicate an error, the transfer of the second
pulse-width-modulated digital signal 13 via the element 12c is blocked.
The computation unit 5 detects the presence of the error state from this
failure. The error detected in this manner can be reported to an operator
or, via the two-wire line 10, 11, to a superordinate control station in
an automation system by appropriately driving an indication unit of the
measurement transducer 1, which unit is not illustrated in the FIGURE for
the sake of clarity. It is possible to completely monitor the generation
of the analog output signal 7 in the output circuit of the measurement
transducer 1 in the manner described. In order to report that the
measurement transducer 1 is in the protected state, the computation unit
5 generates an indication signal 21 which is passed via the element 12d
of the DC-isolation device and drives a current controller 22 for
generating the loop current I such that the loop current I is set to a
value greater than 22.6 mA or less than 3.5 mA.

[0024] The 4 to 20 mA interface of the measurement transducer illustrated
in the figure is additionally able to perform data transmission in
accordance with the HART protocol. In order to transmit data, the
computation unit 5 outputs a digital signal 23 which is passed to a
frequency modulator 24 via the element 12a of the DC-isolation device.
Before the analog output signal is passed to the current controller 22,
the frequency modulated output signal 26 from the frequency modulator 24
is superimposed on the analog output signal with the aid of a summing
device 25. The loop current I therefore consists of an analog part
corresponding to the analog output signal 7 and an AC part corresponding
to the frequency-modulated signal 26. The magnitude of the loop current I
which has been set is detected with the aid of a measuring resistor 27
which can also be used to detect the actual value in the control loop of
the current controller 22. An output signal 28 representing this
magnitude is passed to a subtractor 29 which removes the AC component,
which corresponds to the frequency-modulated signal 26, from the signal
28. As the output signal, the subtractor 29 thus provides the measured
analog output signal 7' which has been freed of the AC part generated in
accordance with the HART protocol. As a result of the loop current I
being detected downstream of the current controller 22, the loop current
I is also advantageously monitored for correct operation. In addition,
the current controller 22 can be supplemented with the monitoring device
known from previously described publication DE 10 2007 059 847 A1. This
document also describes the detailed structure of the current controller
22 which has not been illustrated in the present application for the sake
of clarity. The process of monitoring the generation of the analog output
signal, as described in the present application, can equally be used with
a 4 to 20 mA interface with or without data transmission in accordance
with the HART protocol on account of the use of the subtractor 29.

[0025] A measurement transducer that is a field device is described in the
exemplary embodiment shown. It should be understood that the invention
can also be used in other types of field devices with an analog output,
for example, in actuators such as control valves, or in a programmable
logic controller with an analog output assembly which is suitable, for
example, for connecting actuators via two-wire lines.

[0026] Thus, while there have shown and described and pointed out
fundamental novel features of the invention as applied to a preferred
embodiment thereof, it will be understood that various omissions and
substitutions and changes in the form and details of the devices
illustrated, and in their operation, may be made by those skilled in the
art without departing from the spirit of the invention. For example, it
is expressly intended that all combinations of those elements steps which
perform substantially the same function in substantially the same way to
achieve the same results are within the scope of the invention. Moreover,
it should be recognized that structures and/or elements shown and/or
described in connection with any disclosed form or embodiment of the
invention may be incorporated in any other disclosed or described or
suggested form or embodiment as a general matter of design choice. It is
the intention, therefore, to be limited only as indicated by the scope of
the claims appended hereto.